Iec 62037 6 2013

IEC 62037 6 Edition 1 0 2013 01 INTERNATIONAL STANDARD NORME INTERNATIONALE Passive RF and microwave devices, intermodulation level measurement – Part 6 Measurement of passive intermodulation in anten[.]

Passive RF and microwave devices, intermodulation level measurement –

Part 6: Measurement of passive intermodulation in antennas

Dispositifs RF et à micro-ondes passifs, mesure du niveau d’intermodulation –

Partie 6: Mesure de l’intermodulation passive dans les antennes

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Passive RF and microwave devices, intermodulation level measurement –

Part 6: Measurement of passive intermodulation in antennas

Dispositifs RF et à micro-ondes passifs, mesure du niveau d’intermodulation –

Partie 6: Mesure de l’intermodulation passive dans les antennes

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CONTENTS

FOREWORD 3

1 Scope 5

2 Normative references 5

3 Abbreviations 5

4 Antenna definitions as it pertains to PIM 5

4.1 Antenna 5

4.2 Antenna under test 6

4.3 Active antenna 6

4.4 Antenna PIM 6

5 Antenna design and field installation considerations 6

5.1 Environmental effects on PIM performance 6

5.2 Antenna interface connection 6

5.3 Mounting considerations to avoid PIM generation 6

5.4 Neighbouring sources of interference 7

5.5 Standard practices and guidelines for material selection 7

6 PIM measurement considerations 7

6.1 Quality assurance process and handling procedures 7

6.2 Measurement accuracy 7

6.3 Test environment 8

6.4 Safety 8

6.5 Test set-up 8

6.5.1 Coaxial test cable assemblies 8

6.5.2 Defining a good low PIM reference load 8

6.5.3 Test set-up and test site baseline PIM verification 8

6.6 PIM test configurations 9

6.7 Combined environmental and PIM testing 10

6.7.1 General 10

6.7.2 Mechanical considerations 10

6.7.3 Test system cables and connectors 11

6.8 PIM test chamber design 11

6.8.1 General 11

6.8.2 RF absorber materials 11

6.8.3 Supporting structures and walls 12

6.8.4 RF shielding 12

Figure 1 – Antenna reverse PIM test set-up 9

Figure 2 – Antenna forward PIM test set-up 10

INTERNATIONAL ELECTROTECHNICAL COMMISSION

PASSIVE RF AND MICROWAVE DEVICES, INTERMODULATION LEVEL MEASUREMENT – Part 6: Measurement of passive intermodulation in antennas

FOREWORD

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International Standard IEC 62037-6 has been prepared by technical committee 46: Cables,

wires, waveguides, r.f connectors, r.f and microwave passive components and accessories

This bilingual version (2014-01) corresponds to the monolingual English version, published in

2013-01

The text of this standard is based on the following documents:

Full information on the voting for the approval of this standard can be found in the report on

voting indicated in the above table

The French version of this standard has not been voted upon

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2

A list of all the parts in the IEC 62037 series, published under the general title Passive RF

and microwave devices, Intermodulation level measurement can be found on the IEC website

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data

related to the specific publication At this date, the publication will be

• reconfirmed,

• withdrawn,

• replaced by a revised edition, or

• amended

PASSIVE RF AND MICROWAVE DEVICES, INTERMODULATION LEVEL MEASUREMENT – Part 6: Measurement of passive intermodulation in antennas

1 Scope

This part of IEC 62037 defines test fixtures and procedures recommended for measuring

levels of passive intermodulation generated by antennas, typically used in wireless

communication systems The purpose is to define qualification and acceptance test methods

for antennas for use in low intermodulation (low IM) applications

2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and

are indispensable for its application For dated references, only the edition cited applies For

undated references, the latest edition of the referenced document (including any

amendments) applies

IEC 62037-1:2012, Passive r.f and microwave devices, intermodulation level measurement –

Part 1: General requirements and measuring methods

IEC 62037-3, Passive r.f and microwave devices, intermodulation level measurement –

Part 3: Measurement of passive intermodulation in coaxial connectors

3 Abbreviations

AIM Active intermodulation

AUT Antenna under test

ESD Electrostatic discharge

HPA High power amplifier

IM Intermodulation

LNA Low noise amplifier

PIM Passive intermodulation

RF Radio frequency

4 Antenna definitions as it pertains to PIM

4.1 Antenna

An antenna is that part of a radio transmitting or receiving system which is designed to

provide the required coupling between a transmitter or a receiver and the medium in which the

radio wave propagates

The antenna consists of a number of parts or components These components include, but

are not limited to, one or many radiating elements, one or many RF interfaces, a distribution

or combining feed network, internal support structures, devices which control or adjust the

amplitude/phase response and distribution to the radiating element(s), filters, diplexers,

orthomode transducers, polarizers, waveguides, coaxial cables or printed circuits In addition,

peripheral components could also influence the PIM performance of the antenna These

components may include, but are not limited to, mounting brackets, mounting hardware,

radome, radome fasteners, thermal insulation and grounding hardware

4.2 Antenna under test

The antenna hardware can have an effect on the overall antenna PIM performance

Therefore, it is necessary to specify the hardware which is to be part of the antenna under

test (AUT)

4.3 Active antenna

An active antenna incorporates active devices such as low noise amplifiers (LNAs), high

power amplifiers (HPAs), phase shifters, etc An active antenna has the additional concern of

active intermodulation (AIM) which is typically at a much higher level than PIM The

measurement of PIM in the presence of AIM is not within the scope of this standard If

required, the PIM measurement of an active antenna shall be performed on the passive

portion of the antenna only

4.4 Antenna PIM

The antenna PIM is defined as the PIM that is generated by the antenna assembly itself at a

reference plane or RF interface The PIM can be measured in a radiated or conducted

(transmissive or reflective) mode

5 Antenna design and field installation considerations

5.1 Environmental effects on PIM performance

Any hardware located in the near-by environment can significantly influence the PIM

performance of an antenna or antenna system The effect of ferromagnetic materials,

dissimilar metallic junctions which are part of neighbouring hardware, such as other antennas,

towers structures, aircraft fuselage components, spacecraft thermal control hardware, d.c

and ESD grounding hardware, non-high pressure mechanical connections etc., can potentially

have a detrimental effect on the PIM performance of the communication system

5.2 Antenna interface connection

Any interface that is exposed to RF is a potential PIM source and shall be designed to be low

PIM Care shall be taken to ensure that all the mating surfaces are clean The connections,

whether coaxial or waveguide, should be inspected for dirt, metallic filings, sharp protruding

material, and other potential contaminates Any coaxial connections shall be torqued to the

manufacturer’s specifications to assure proper metal-to-metal contact pressure is achieved If

waveguide is used, then the flange bolts shall be torqued to the recommended manufacturer’s

specifications Careful attention shall be paid to the alignment of the mating coaxial

connectors or waveguide flanges

The materials and combination of materials used in the connectors, including plating, are

important for the PIM performance The use of a soft plating material (e.g gold, silver, etc.) of

sufficient thickness (several skin depths) over a hard base material (brass, BeCu, etc.) is

usually preferable The number of interfaces (coaxial connectors and adapters) should be

minimized This will reduce the number of metal-to-metal junctions and, thus, the possibility of

PIM generation More information about coaxial connectors can be found in IEC 62037-3

5.3 Mounting considerations to avoid PIM generation

The antenna shall be properly secured to its mounting bracket All bolts and holding

harnesses used to secure the antenna to its support structure shall be tightened and torqued

according to the manufacturer’s specifications The coaxial or waveguide transmission line(s)

leading to the antenna input port(s) shall also be well-secured and prohibited from rubbing or

moving

Care should be taken in the antenna placement by pointing it towards a clear sky view and to

isolate it from all possible neighbouring sources of interference such as tower structures,

near-by antennas, buildings, walls, aircraft fuselage, spacecraft platform, etc

5.4 Neighbouring sources of interference

Knowledge of the RF environment in which the antenna is to be installed is important Care

should be taken in the antenna placement to isolate it from all possible neighbouring sources

of interference For instance, structures having low contact pressure or corroding parts should

be avoided Additionally, other antennas radiating in a similar band or in bands whose

harmonics could fall within the receive frequency band of the antenna being installed also

requires consideration Other electric or electronic devices may emit interfering RF signals

that fall into the receive frequency band of the antenna

5.5 Standard practices and guidelines for material selection

Clause 6 of IEC 62037-1:2012 serves as a guide for the design, selection of materials, and

handling of components that may be susceptible to PIM generation It is very important to

consider the application of the antenna, as there are large differences in acceptable PIM

levels between space applications and terrestrial applications

6 PIM measurement considerations

6.1 Quality assurance process and handling procedures

The purpose of Clause 6 is to provide guidance in the areas of quality control as it pertains to

the performance of PIM testing of antenna products Procedures are included to enhance the

accuracy and ensure safety when performing PIM measurements on antenna products The

following guidelines will help minimize errors induced within the test system

6.2 Measurement accuracy

The accuracy of PIM tests performed on antenna products may be severely affected by a

multitude of sources that may be either external or internal to the test system Some of the

sources which can affect the results of PIM tests performed on antenna products include, but

are not limited to, the following:

a) objects comprising parts made of electrically conductive materials that are exposed to the

electromagnetic fields radiated by the AUT;

b) loose, damaged or corroded mounting hardware attached to the AUT;

c) loose or corroded hardware exposed to the radiated RF fields from the AUT;

d) radio frequency signals generated by external sources;

e) faulty or poorly performing coaxial interface cables;

f) dirty/contaminated/worn interface connections;

g) improperly mated interface connections;

h) poorly shielded RF interface connections;

i) inadequately filtered AIM from the test set-up;

j) consideration should be given to input transmission line losses;

k) contaminated absorbers

6.3 Test environment

When applicable, PIM measurements may be accomplished outdoors In performing such a

test, it is important to ensure that government regulations pertaining to the maximum

authorized RF radiation levels are met Also, the RF energy radiated from the AUT may

generate PIM in surrounding structures that may couple back into the antenna resulting in

invalid PIM test results Additionally, external sources of RF radiation may interfere with the

test measurements A survey of the frequencies locally in use is recommended prior to

testing Many of the external sources of PIM may be minimized or eliminated by performing

the PIM testing of antennas within an anechoic test chamber providing a low PIM test

environment More information on the construction of anechoic test chambers suitable for PIM

testing is provided in 6.8

6.4 Safety

Performing PIM tests on antenna products can be dangerous Potentially high voltages and

high levels of RF energy may be present both within the AUT and within the test environment

The AUT should be positioned such that personnel will not be exposed to electromagnetic

fields exceeding the acceptable levels specified by government agencies

6.5 Test set-up

A problem with PIM test set-ups using coaxial cable interfaces is the need to repeatedly

connect/disconnect coaxial connectors The following are some recommendations on test

set-up procedures

a) Sealing O-rings at connector interfaces should be thoroughly cleaned or should preferably

be avoided if possible These O-rings accumulate metal filings, which can become a

source of PIM

b) Inspect connectors, dielectric and interface mating surfaces or flanges for contamination,

especially metallic debris, just prior to mating the interface Also inspect connector mating

surfaces for burrs, scratches, dents, and loss of plating Proper installation and torquing of

the hardware will minimize the generation of PIM within interface connections

c) Clean compressed air should be used to blow potential metal particles from the connector

interfaces after each connect-disconnect cycle

d) Great care shall be taken to ensure that the cables have not been stressed or fatigued to

the point of cracking The inner and outer conductors can crack under the insulating cable

jacket and not be detectable by visual inspection This will cause intermittent PIM signals

to be generated One way to test for this is to flex or tap on the cable while performing a

baseline test If there are fluctuations in the PIM signal, the cable may be damaged and

should be replaced

A good low PIM load can be made using a long section of high quality coaxial cable

terminated with a high quality (low PIM) connector This connector should be soldered to the

coaxial cable on both the inner and outer conductors The length of cable should be held in a

fixture so that no fatigue is placed on the connector or cable

Prior to the testing of the antenna, perform a baseline PIM test set-up noise floor verification

To verify the test set-up itself, a low PIM termination may be used Check the cables and

connections for sensitivity to flexure, mechanical stress and configuration during the baseline

test

The test site should also be evaluated to ensure that it does not generate unacceptable levels

of PIM or to identify any potential extraneous interfering RF sources The test site could be an

anechoic test enclosure or a chosen outdoor site If an anechoic chamber is used, special

design considerations are needed as outlined in 6.8 During the site verification, if possible,

use a low PIM reference antenna having a radiation pattern and gain comparable to that of

the AUT in order to ensure that the test environment is exposed to representative flux

densities as for the AUT test

The actual antenna PIM test should be performed using the same set-up as for the baseline

test: minimize movements of components, do not add components, minimize changes in the

environment, etc After the antenna PIM test is completed or as required during the test,

compare the baseline test results with previous set-up verification results for any sign of

degradation in the test system

6.6 PIM test configurations

A typical test set-up for antenna reverse (reflected) PIM testing is shown in Figure 1 and one

for antenna forward (transmitted) PIM is shown in Figure 2 It should be noted that dynamic

range between the two test configurations should be examined to assess the appropriate

choices to use In both cases, the test should take place in either a well-designed low PIM

anechoic chamber or outdoors, which would allow full range of antenna movement For the

antenna forward (transmitted) PIM test, a low PIM antenna on the receiver side of the test

set-up is required Also for this test, the environment may be first verified by using two low PIM

antennas

Whenever possible, the diplexer (Figure 1) and the filter (Figure 2), both of which should be

low PIM, shall be placed as close as possible to the AUT input port to minimize PIM

generated by the test set-up The overall cable or waveguide lengths should be minimized to

deliver maximum power to the AUT Also, coaxial and waveguide adapters should be avoided

Test chamber Amplifier Transmit filter

Each set-up has two synthesized sources, amplified separately to avoid AIM (active

intermodulation) The two-tone-test results in discrete intermodulation products, whose levels

are to be measured These PIM-products are typically first amplified by one or two stages of

LNAs before detection by the spectrum analyser or digital receiver This is in order to increase

the sensitivity of the set-up

amplifier

Amplifier Transmit filter

Whenever possible and practical, each AUT should be measured for PIM while being exposed

to representative environmental operating conditions If it is not possible, the AUT may be

measured for PIM before and after exposure to representative environmental conditions

A loose mechanical joint is likely to cause PIM Materials expand and contract due to

temperature changes Different materials expand and contract at different rates This

difference can cause varying amounts of stress to be induced in any mechanical joint of the

antenna components The differences in expansion and contraction can even cause the parts

to move so much as to loosen a mechanical joint A bolted joint that was torqued to its

specified value can loosen to the point where the required clamping force is no longer being

produced Evaluation of mechanical connections may be accomplished by performing PIM

testing during thermal cycling

Vibrations can produce detrimental effects similar to those from thermal environments

For terrestrial applications, extreme temperature cycling occurs only in specific geographical

areas and is more applicable to aeronautical and space applications Wind-induced vibrations

occur in most terrestrial and aeronautical applications but never for space applications

However, vibrations are induced on space-borne antennas during platform manoeuvres For

space and aeronautical applications, it is recommended that PIM testing be performed during

thermal cycling before and after vibration testing

The test cables connected to the antenna under test are exposed to the same test

environments as the antenna itself Therefore, great care shall be taken in selecting cables

suitable for PIM testing in the specific test environment The entire test set-up, including the

cables, shall be verified under the same test conditions as for the AUT testing

6.8 PIM test chamber design

The purpose of 6.8 is to provide guidance for the construction of test chambers suitable for

the performance of PIM testing on antennas

Evaluation of antenna products for PIM presents additional challenges not found with other

non-radiating components The antenna will be connected to an RF source and will radiate RF

energy during the PIM test This energy shall not be allowed to excite potential PIM sources in

the test environment It is also sometimes not practical to perform these tests in an outdoor

environment since the radiated RF energy should preferably be contained To successfully

perform PIM testing on antennas, it may be desirable to construct an RF anechoic chamber

specially designed for PIM testing

The main components of an RF anechoic test chamber are:

RF absorber materials are commonly manufactured from a carbon impregnated foam This

material offers attenuation to radio frequency signals as they pass through it This attenuation

of the signal (absorption of energy) serves in essence as a “load” to the antenna

RF absorber materials are available in many styles and sizes The selection of style and size

is dependent on the frequency of operation and the placement within the test chamber Proper

selection of the RF absorbers may be the most critical factor in the construction of a PIM test

chamber Recommendations that may help in the selection process are as follows

a) Select an absorber with an incident RF attenuation greater than 30 dB

b) For good results, place pyramidal absorber panels in the field of the antenna radiation

pattern, preferably with normal incidence to the beam peak However, best results can be

achieved when the interior of the test chamber is completely covered with RF absorber

material

c) As a minimum, ensure there are enough panels to avoid back reflections

For safety purposes, select an absorber that contains fire retardant materials and is rated for

the anticipated maximum power dissipation required

6.8.3 Supporting structures and walls

The supporting structure and walls for the PIM test chamber shall provide a suitable inner

surface for attachment of the RF absorber material In some applications, the supporting

structure and walls may also be required to assist in the control of the temperature, the

pressure, the humidity level, or other environmental conditions for the test

The materials and methods of construction will vary greatly depending on the specific

application For many applications, simple lumber and plywood provide very good results

Cement block construction also provides excellent support but at a much greater expense

Some general considerations in designing the support structure and walls are as follows

a) The use of metal shielding in the outer structure improves the isolation of the anechoic

chamber and is recommended when RF shielding needs to be high (see 6.8.4) However,

it is critical to ensure that the design does not include metal-to-metal junctions that

themselves have poor PIM performance Examples of this would include overlapping metal

plates or the use of metal hardware going through sheet metal parts that are exposed

b) Wood supports can be successfully joined using screws Screws are stronger than nails

and it is easier to control their final location Do not allow metal fasteners to contact each

other, even within the framework

c) Make sure that the actual dimensions of absorber panels are known before completing the

design of the structure as they do not usually have the exact size advertised

d) The size of the test chamber should be large enough to allow the test antenna to be

sufficiently far from any RF absorber to avoid mutual coupling between the radiating

antenna and the absorber material

e) Hinges, fasteners, light fixtures, fire sprinklers, mounting hardware, etc should all be

evaluated for potential PIM generation

RF shielding may or may not be required, depending on the particular application The

purpose of RF shielding may be for security, to maintain a low RF noise floor in the test

facility, or may be required to ensure personnel safety A method of identifying the need for

RF shielding is based on the calculated power densities From such calculations, it may be

found that RF levels behind the RF absorber are extremely low and therefore safe It is

always recommended that an RF survey of the area surrounding the chamber be performed

prior to the approval of the final test plan or procedure

Methods of RF shielding also vary depending on the application One method providing good

results for most applications is to apply thin aluminium sheets or panels to the exterior surface

of the test chamber structure The sheets can be securely attached using adhesive products

Placing a plastic insulating material on the edge of each panel will prevent any direct contact

between panels A small gap between the panels will not pass RF energy except at extremely

small wavelengths compared to the gap size Although RF power levels may be extremely low

at the RF shield, it would still be advisable to avoid materials which may generate PIM such

as wire mesh fabrics

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